Some animals spend their lives in constant motion – dashing, jumping, flitting, swimming, running, swinging and soaring about – while others are veritable couch potatoes. Differences in daily physical activity can occur because of ecological disparities, such as relative predation risk or proximity to resources; or internal factors, such as individual needs, motivations and physical limitations. There is also another reason why individuals vary in the amount of time they spend moving – personality. Animal personalities are generally thought to vary in terms of how willing they are to explore, their boldness, aggressiveness and sociability. Regular exercise has been shown to elicit increased motivation to continue exercising (i.e. it changes behavior). Elektra Sinclair, of the University of Sydney, and her international team of collaborators asked the question: if exercise can change some behaviors, can it also affect personality? Could the personalities of the mosquitofish she studied be altered by changing the amount of time they were required to exercise each day, i.e. would shy, sedentary fish be molded into lean, mean, venturous machines?

Sinclair and her colleagues caught wild mosquitofish and housed some of them in tanks where water flowed continuously (exercise groups), and some in tanks where the water was still (control groups). After 28 days, they tested the maximum swimming speed and strength of the fish in each group, and how quickly each fish left a novel refuge, as well as the distance the fish explored throughout the tank. Lastly, the authors placed similarly sized pairs of males within each treatment group together and recorded the behavior of the males when they came in contact with each other, to determine which group of males was more aggressive towards other males. Sinclair and her team found that not only were the fish that had been undergoing fitness training for a month significantly better swimmers, relative to the sedentary group, but also they were bolder, explored more and were more aggressive with other males.

To test whether it was the increased locomotor performance that influenced the behavior of the fish (that is, enhanced performance gave them a boost that encouraged boldness), the authors repeated the treatments (with exercise and without) with new sets of fish, but before the behavioral tests, they administered a calcium channel blocker (nifedipine) to the exercised fish to reduce swimming performance. As expected, the nifedipine-treated fit fish did not out-swim the sedentary fish. And, following the treatment, the fit fish were no more eager to explore or aggressive than the control fish. Surprisingly, however, the nifedipine had no effect on boldness. The nifedipine-treated fit fish fearlessly continued to leave their refuges sooner than the control fish.

By forcing the fish to swim regularly, the authors improved the fish's overall locomotor performance via increased cardiovascular, muscular and metabolic function, releasing them from the physiological constraint imposed by inactivity. The interplay between ecology, physiology and behavior demonstrated in this study has far-reaching implications. Not only are differences in personality at the population level potentially mediated by the specific physical demands of the local habitat but also animal personalities are labile and reversible. In addition, the training effects on personality should not be ignored in studies of performance. This study also suggests that the best way to get, and stay, motivated is to start moving. So start moving.

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